Solid electrolytic capacitor and method of manufacturing thereof

ABSTRACT

A solid electrolytic capacitor with suppressed occurrence of short circuit is provided. The solid electrolytic capacitor includes an anode body having a surface on which a dielectric film is formed, and a conductive polymer layer formed on the dielectric film. The conductive polymer layer includes at least a first conductive polymer layer formed on the dielectric film and a second conductive polymer layer formed on the first conductive polymer layer. A silane compound in the first conductive polymer layer and the silane compound in the second conductive polymer layer have respective concentrations different from each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 12/862,319,filed Aug. 24, 2010, which is a nonprovisional application based onJapanese Patent Application No. 2009-198240, filed on Aug. 28, 2009 withthe Japan Patent Office, the entire contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid electrolytic capacitor and amethod of manufacturing the solid electrolytic capacitor, andparticularly to a solid electrolytic capacitor with suppressedoccurrence of short circuit and a method of manufacturing the solidelectrolytic capacitor.

2. Description of the Related Art

Recently, electronic devices have been downsized and increased infrequency, which also requires downsizing and increased frequency of acapacitor that is an electronic component of an electronic device. Anexample of capacitors suitable for being downsized is a solidelectrolytic capacitor including an anode body of a valve metal, adielectric film formed on the anode body, and a layer of manganesedioxide or a conductive polymer for example formed on the dielectricfilm.

In the solid electrolytic capacitor, the dielectric film is producedthrough a chemical conversion performed on the valve metal of which theanode body is formed. The dielectric film produced in theabove-described manner is extremely dense, highly durable, and verythin. The solid electrolytic capacitor can therefore be downsizedwithout being reduced in capacitance, as compared with other types ofcapacitors such as paper capacitor and film capacitor.

Although such a solid electrolytic capacitor can be downsized, thedownsized capacitor tends to encounter increased occurrences of leakagecurrent (LC) and short circuit in the manufacturing process. Even afterthe capacitor has been manufactured, a high-temperature reflow processtends to cause LC.

With the purpose of suppressing such occurrence of short circuit,Japanese Patent Laying-Open No. 2005-109252 for example discloses thefollowing method. An anode body on which a dielectric film is formed isimmersed in a polymerizable monomer solution and dried so that aconductive polymer layer is formed, and thereafter further immersed in aconductive polymer solution (or conductive polymer solution is applied)and dried. According to Japanese Patent Laying-Open No. 2005-109252,this method can uniformly form the conductive polymer layer and therebysuppress occurrence of short circuit.

Currently, however, the need to suppress occurrence of short circuitstill remains. Further, in addition to and simultaneously withsuppression of occurrence of short circuit, maintenance of capacitor'sintrinsic characteristics such as maintenance of the capacitance andsuppression of increase in ESR (Equivalent Series Resistance), forexample, are required.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a solid electrolyticcapacitor having suppressed occurrence of short circuit while keepingcapacitor's intrinsic characteristics, and a method of manufacturing thesolid electrolytic capacitor.

The inventors of the present invention have focused on the concentrationof a silane compound in the conductive polymer layer of the solidelectrolytic capacitor and conducted serious studies, and finally foundthat the concentration of the silane compound can be varied along thethickness direction of the conductive polymer layer to suppressoccurrence of short circuit while capacitor's intrinsic characteristicsare maintained. The present invention is based on this finding.

Specifically, the present invention is a solid electrolytic capacitorincluding an anode body having a surface on which a dielectric film isformed, and a conductive polymer layer formed on the dielectric film.The conductive polymer layer includes at least a first conductivepolymer layer formed on the dielectric film and a second conductivepolymer layer formed on the first conductive polymer layer. The firstconductive polymer layer and the second conductive polymer layer containa silane compound. The silane compound in the first conductive polymerlayer and the silane compound in the second conductive polymer layerhave respective concentrations different from each other.

Regarding the above-described solid electrolytic capacitor, preferablythe concentration of the silane compound in the first conductive polymerlayer is higher than the concentration of the silane compound in thesecond conductive polymer layer.

Regarding the above-described solid electrolytic capacitor, preferablythe anode body is formed of a sintered body.

The present invention is a method of manufacturing a solid electrolyticcapacitor including an anode body having a surface on which a dielectricfilm is formed, and a conductive polymer layer formed on a surface ofthe dielectric film. The method includes the steps of forming thedielectric film on the anode body, and forming the conductive polymerlayer on the dielectric film. The step of forming the conductive polymerlayer includes at least the steps of forming a first conductive polymerlayer on the dielectric film by using a first polymerization solutioncontaining a silane compound, and forming a second conductive polymerlayer on the first conductive polymer layer by using a secondpolymerization solution containing a silane compound. The silanecompound in the first polymerization solution and the silane compound inthe second polymerization solution have respective concentrationsdifferent from each other.

Regarding the above-described method of manufacturing a solidelectrolytic capacitor, preferably the concentration of the silanecompound in the first polymerization solution is higher than theconcentration of the silane compound in the second polymerizationsolution.

Regarding the above-described method of manufacturing a solidelectrolytic capacitor, preferably the concentration of the silanecompound in the first polymerization solution and the secondpolymerization solution is not less than 5% and not more than 30%.

Regarding the above-described method of manufacturing a solidelectrolytic capacitor, preferably the anode body is formed of asintered body.

“Precursor monomer” herein may not necessarily be a monomer and mayinclude, for example, low-molecular-weight oligomers, and “oxidizer”herein may function as a dopant. Further, “concentration of substance A”in a polymerization solution constituted of substances A to C hereinrefers to the ratio X to Y+Z where X to Z represents respective weightsof substances A to C, respectively.

The present invention can provide a solid electrolytic capacitor havingsuppressed occurrence of short circuit while keeping capacitor'sintrinsic characteristics as well as a method of manufacturing the solidelectrolytic capacitor.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section schematically showing a structure of a solidelectrolytic capacitor according to an embodiment.

FIGS. 2A to 2E are process diagrams illustrating a method ofmanufacturing a solid electrolytic capacitor according to an embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will hereinafter be describedbased on the drawings. In the drawings referenced below, the same orcorresponding components are denoted by the same reference characters,and a description thereof will not be repeated. It should be noted thatthe relation between the components in terms of the dimension such aslength, size, width or the like in the drawings has been changed asappropriate for the sake of clarification and simplification of thedrawings, namely the dimension as shown does not represent the actualdimension.

Structure of Solid Electrolytic Capacitor

Referring to FIG. 1, a solid electrolytic capacitor 100 includes acapacitor element 10 that includes an anode body 11 having a surface onwhich a dielectric film 12 is formed, a conductive polymer layer 13formed on dielectric film 12, and a carbon layer 14 and a silver pastelayer 15 that are formed on conductive polymer layer 13 and serve ascathode lead layers.

Solid electrolytic capacitor 100 further includes an anode lead 16, ananode terminal 17, an adhesive layer 18, and a cathode terminal 19.Anode lead 16 is a rod-shaped body made of a metal such as tantalum forexample, has one end embedded in anode body 11 and the other end placedto protrude on the outside of capacitor element 10. Anode terminal 17has a part connected by welding to anode lead 16. Cathode terminal 19 isplaced to connect to silver paste layer 15, which is the outermost layerof capacitor element 10, via adhesive layer 18 formed of an electricallyconductive adhesive.

Solid electrolytic capacitor 100 further includes a coating resin 20.Coating resin 20 seals capacitor element 10 for which anode lead 16,anode terminal 17, adhesive layer 18, and cathode terminal 19 arearranged, in such a manner that exposes a part of anode terminal 17 anda part of cathode terminal 19 on coating resin 20.

In above-described solid electrolytic capacitor 100, anode body 11 isformed of a sintered body of a valve action metal (tantalum, niobium,titanium, aluminum, or the like), and dielectric film 12 is an oxidefilm formed through a chemical conversion performed on the valve actionmetal. For example, when tantalum (Ta) is used as the valve actionmetal, the composition of dielectric film 12 is Ta₂O₅ and, when aluminum(Al) is used as the valve action metal, the composition of dielectricfilm 12 is Al₂O₃. The sintered body here has a porous structure.

Conductive polymer layer 13 is constituted to include a conductivepolymer and a silane compound. This conductive polymer layer 13 isformed on dielectric film 12 by, for example, immersing anode body 11having its surface on which dielectric film 12 is formed, in apolymerization solution containing a precursor monomer of a conductivepolymer, a silane compound, and an oxidizer. Alternatively, for example,anode body 11 on which dielectric film 12 is formed may be immersed in apolymerization solution containing a precursor monomer and a silanecompound and thereafter immersed in a solution containing an oxidizer.Instead, for example, anode body 11 on which dielectric film 12 isformed may be immersed in a solution containing an oxidizer andthereafter immersed in a polymerization solution containing a precursormonomer and a silane compound. Instead, on anode body 11 on whichdielectric film 12 is formed, a polymerization solution containing aprecursor monomer, a silane compound, and an oxidizer may be applied.

As to the silane compound, the silane compound that is chemically bondedto an inorganic matter through hydrolysis and dehydration can protect adefect of the dielectric film. In view of such a function of the silanecompound, it appears that the concentration of the silane compound inthe conductive polymer layer can be increased to suppress occurrence ofshort circuit of the solid electrolytic capacitor. The inventors of thepresent invention, however, have found that the increased concentrationof the silane compound, relative to the conventional concentrationthereof, in conductive polymer layer 13 suppresses occurrence of shortcircuit of solid electrolytic capacitor 100 while it disadvantageouslycauses increase of ESR. A reason for this appears to be that the silanecompound itself is an insulator.

In order to achieve both of suppression of occurrence of short circuitand suppression of increase of ESR, the inventors of the presentinvention have conducted serious studies to succeed in achieving both ofsuppression of occurrence of short circuit and suppression of increaseof ESR, by varying the concentration of the silane compound along thethickness direction of conductive polymer layer 13.

More specifically, in solid electrolytic capacitor 100 of the presentembodiment, the concentration of the silane compound in conductivepolymer layer 13 is not uniform but varied in a stepwise manner alongthe thickness direction. A form of such a conductive polymer layer isshown in FIG. 1. Namely, conductive polymer layer 13 is constituted of afirst conductive polymer layer (inner layer) having a surface 13 a and asecond conductive polymer layer (outer layer) having a surface 13 b andformed on the first conductive polymer layer, and the concentration ofthe silane compound in the first conductive polymer layer and theconcentration of the silane compound in the second conductive polymerlayer are different from each other. In this way, occurrence of shortcircuit of the solid electrolytic capacitor can be suppressedeffectively.

When conductive polymer layer 13 has a double layer structure asdescribed above, preferably the concentration of the silane compound inthe first conductive polymer layer is higher than the concentration ofthe silane compound in the second conductive polymer layer. In thiscase, occurrence of short circuit of solid electrolytic capacitor 100can more effectively be suppressed.

Further, as will be understood, the structure of conductive polymerlayer 13 is not limited to the double layer structure and may beconstituted of any number of layers. In particular, it is preferablethat a conductive polymer layer having surface 13 a which is theinnermost layer among the layers constituting conductive polymer layer13, has the concentration of the silane compound higher than theconcentration of the silane compound in another layer or other layers,particularly a conductive polymer layer having surface 13 b which is theoutermost layer. In this case, occurrence of short circuit of the solidelectrolytic capacitor can more effectively be suppressed.

When conductive polymer layer 13 is constituted of a plurality oflayers, the precursor monomer, the silane compound, the type of theoxidizer, and the type of a solvent containing them, which are used informing each layer, may be different between the layers.

The conductive polymer which is a component of conductive polymer layer13 is preferably a polymer having at least one of aliphatic-basedcompounds, aromatic-based compounds, heterocyclic-based compounds, andheteroatom-contained compounds. In particular, polythiophene,polypyrrole, polyaniline, polyfuran, TCNQ(7,7,8,8-tetracyanoquinodimethane) silver salt, or the like, ispreferred.

Examples of the silane compound which is another component of conductivepolymer layer 13 include, for example, vinyltrichlorosilane,vinyl(β-methoxysilane), vinyltriethoxysilane, γ-methacryloxysilane,β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,γ-glycidoxypropyltrimethoxysilane,N-β-(aminoethyl)-γ-aminopropylmethyldimethoxysilane,N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane,γ-aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane,γ-mercaptopropyltrimethoxysilane, γ-chloropropyltrimethoxysilane, andthe like.

In particular, it is preferable to use, as the silane compound,γ-glycidoxypropyltrimethoxysilane orN-phenyl-γ-aminopropyltrimethoxysilane. They are excellent in terms ofsuppression of occurrence of short circuit and suppression of increaseof ESR of solid electrolytic capacitor 100.

An example of the oxidizer is a sulfonic acid metal salt. The sulfonicacid metal salt serves as an oxidizer and as a dopant. The sulfonic acidmay be alkyl sulfonic acid, aromatic sulfonic acid, polycyclic aromaticsulfonic acid, or the like, and the metal may be a metal selected asappropriate from iron (III), copper (II), chromium (IV), cerium (IV),ruthenium (III), zinc (II), and the like.

Carbon layer 14 serving as the cathode lead layer may at least haveconductivity and, may be formed using graphite. Anode terminal 17 andcathode terminal 19 may be formed using a metal such as copper or copperalloy, for example. Epoxy resin for example may be used as a materialfor coating resin 20.

Method of Manufacturing Solid Electrolytic Capacitor

A description will now be given, using FIGS. 2A to 2E, of a method ofmanufacturing solid electrolytic capacitor 100. Here, by way of example,a method of manufacturing a solid electrolytic capacitor includingconductive polymer layer 13 constituted of a first conductive polymerlayer and a second conductive polymer layer will be described.

Referring first to FIG. 2A, a valve action metal powder is prepared, andan end in the longitudinal direction of rod-shaped anode lead 16 isembedded in the metal powder. In this state, the powder is formed into adesired shape. This is sintered to form anode body 11 of a porousstructure in which the end of anode lead 16 is embedded.

Next, as shown in FIG. 2B, anode body 11 undergoes a chemical conversionso that dielectric film 12 is formed on anode body 11. A method for thechemical conversion may be as follows. Anode body 11 is immersed in aforming solution such as an aqueous solution of phosphoric acid of 0.01to 2% by mass or an aqueous solution of nitric acid, and a voltage isapplied to anode body 11.

Next, anode body 11 on which dielectric film 12 is formed is immersed ina polymerization solution containing a precursor monomer, which is aprecursor of a polymer forming conductive polymer layer 13, a silanecompound, and an oxidizer, and conductive polymer layer 13 is formed ondielectric film 12. At this time, the concentration of the silanecompound in the polymerization solution is adjusted such that theconcentration of the silane compound in conductive polymer layer 13varies in a stepwise manner along the thickness direction of conductivepolymer layer 13, as described below.

First, in a polymerization solution A that is a first polymerizationsolution containing a precursor monomer, a silane compound, and anoxidizer, anode body 11 on which dielectric film 12 is formed isimmersed. Then, anode body 11 taken out of polymerization solution A issubjected to a dry process and the solvent is removed. Thus, as shown inFIG. 2C, an inner layer 13A that is a first conductive polymer layer isformed. For the dry process, heat treatment may be used.

Next, in a polymerization solution B that is a second polymerizationsolution different from polymerization solution A in concentration ofthe silane compound, anode body 11 on which inner layer 13A is formed isimmersed. Anode body 11 taken out of polymerization solution B issubjected to a dry process and the solvent is removed. Thus, as shown inFIG. 2D, an outer layer 13B that is a second conductive polymer layer isformed on inner layer 13A. For the dry process, heat treatment may beused.

As polymerization solution A and polymerization solution B, a solvent,for example, an organic solvent such as 1-butanol in which a precursormonomer, a silane compound, and an oxidizer are contained may be used.Polymerization reaction may be chemical oxidative polymerization,electrochemical polymerization, or the like. The chemical oxidativepolymerization, with which the manufacturing process can be simplifiedand the concentration of the silane compound in conductive polymer layer13 can be easily adjusted, is preferably used.

After this, on anode body 11 having been processed in theabove-described manner, carbon layer 14, silver paste layer 15, anodeterminal 17, adhesive layer 18, and cathode terminal 19 are arranged andsealed with coating resin 20, following a known technique. Accordingly,solid electrolytic capacitor 100 shown in FIG. 2E is produced.

According to the method of manufacturing a solid electrolytic capacitorin the present embodiment, conductive polymer layer 13 is formed throughrepeated immersion and drying processes and, among respectivepolymerization solutions used for the immersion processes, at least onepolymerization solution has a concentration of a silane compounddifferent from a concentration of the silane compound in anotherpolymerization solution. Therefore, when conductive polymer layer 13 ofa double layer structure for example is to be formed, conductive polymerlayer 13 constituted of inner layer 13A and outer layer 13B that aredifferent in concentration of the silane compound may be formed, asshown in FIG. 2E. In this way, increase of ESR due to an excessiveincrease of the amount of the silane compound in conductive polymerlayer 13 can be suppressed and, at the same time, occurrence of shortcircuit can be suppressed.

Further, when conductive polymer layer 13 has a double layer structureas described above, preferably the concentration of the silane compoundin inner layer 13A is higher than the concentration of the silanecompound in outer layer 13B. In this case, occurrence of short circuitcan more effectively be suppressed.

As to the concentration of the silane compound in polymerizationsolutions A and B, the ratio of the weight of the silane compound to theweight of substances other than the silane compound in polymerizationsolutions A and B is preferably not less than 5% and not more than 30%.In this case, effective suppression of increase of ESR due to the amountof the silane compound, as well as suppression of occurrence of shortcircuit can be achieved.

For conductive polymer layer 13 constituted of a plurality of layers,preferably the concentration of the silane compound in polymerizationsolution A, which is used for forming inner layer 13A having surface 13a abutting on dielectric film 12, is made higher than the concentrationof the silane compound in polymerization solution B, which is used forforming an outer layer having surface 13 b abutting on carbon layer 14.In this way, increase of ESR and occurrence of short circuit can moreeffectively be suppressed.

As seen from above, the present invention uses a plurality ofpolymerization solutions for forming a conductive polymer layerconstituted of a plurality of layers, and at least one of thepolymerization solutions has a concentration of the silane compounddifferent from that of another or other polymerization solution(s).Accordingly, the solid electrolytic capacitor to be manufactured cankeep capacitor's intrinsic characteristics, or can have improvedcapacitor's intrinsic characteristics, and occurrence of short circuitcan be suppressed. Further, a plurality of polymerization solutionshaving respective concentrations of the silane compound that are variedin a stepwise manner can be used to form the conductive polymer layer inwhich the concentration of the silane compound varies in a stepwisemanner.

The solid electrolytic capacitor of the present invention is not limitedto the solid electrolytic capacitor in the embodiment as describedabove, but may be applied to known forms. Specific examples of the knownforms may include a wound solid electrolytic capacitor, a stack-typesolid electrolytic capacitor using plates of a valve metal, and thelike.

It should be noted that, in a solid electrolytic capacitor having ananode body formed of a sintered body of a porous structure, a conductivepolymer layer is formed on a dielectric film of the anode body generallyby repeating multiple times the step of immersing the anode body in apolymerization solution and drying it. It is therefore easy in terms ofthe manufacturing process to form the conductive polymer layer of thesolid electrolytic capacitor having the anode body of a sintered body,so that the conductive polymer layer has a double layer structure forexample and an inner layer and an outer layer of the conductive polymerlayer have different silane-compound concentrations from each other.Accordingly, it is preferable to apply the present invention to a solidelectrolytic capacitor having an anode body formed of a sintered body.

EXAMPLES

In the following, the present invention will be described in more detailwith reference to examples. The present invention, however, is notlimited to them.

Example 1

For Example 1, a solid electrolytic capacitor having the structure shownin FIG. 2E was produced. First, a tantalum powder was prepared. An end,in the longitudinal direction, of rod-shaped anode lead 16 was embeddedin the metal powder. In this state, the powder was shaped into arectangular parallelepiped. The resultant powder was sintered to prepareanode body 11 in which the end of anode lead 16 was embedded. Then,anode body 11 was immersed in a phosphoric acid solution of 0.02% bymass, to which a voltage of 100 V was applied. Thus, dielectric film 12of Ta₂O₅ was formed on a surface of anode body 11.

Subsequently, polymerization solution A was prepared by mixing, in1-butanol, 3,4-ethylenedioxythiophene as a precursor monomer,γ-glycidoxypropyltrimethoxysilane as a silane compound, and iron (III)p-toluenesulfonate as an oxidizer having the function of dopants.Polymerization solution A was prepared so that the ratio by weightbetween the compounds, namely3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol, was 1:0.9:5:11.5. Then, anode body 11 onwhich dielectric film 12 was formed was immersed in this polymerizationsolution A for one minute. After this, anode body 11 was taken out ofpolymerization solution A and heat-treated, so that inner layer 13A ofconductive polymer layer 13 was formed.

Subsequently, anode body 11 having inner layer 13A formed thereon wasimmersed in polymerization solution B prepared so that the ratio byweight between the compounds was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:1.5:5:8.5. After this, anode body 11 wastaken out of polymerization solution B and heat-treated, so that outerlayer 13B of conductive polymer layer 13 was formed.

On anode body 11 having been dried, a suspension of graphite particleswas further applied, and the anode body was dried in the atmosphere sothat carbon layer 14 was formed on outer layer 13B. Further, following aknown technique, silver paste layer 15, anode terminal 17, adhesivelayer 18, and cathode terminal 19 were arranged, and they were sealedwith coating resin 20 as shown in FIG. 2E. In this way, the solidelectrolytic capacitor was manufactured.

Example 2

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:1.8:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.7:5:8.5.

Example 3

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:2.6:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.7:5:8.5.

Example 4

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:3.5:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.7:5:8.5.

Example 5

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:5.3:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.7:5:8.5.

Comparative Example 1

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.04:5:11.5, and polymerization solutionB was prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.03:5:8.5.

Comparative Example 2

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.18:5:11.5, and polymerization solutionB was prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.15:5:8.5.

Comparative Example 3

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.7:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.6:5:8.5.

Comparative Example 4

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.9:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:0.7:5:8.5.

Comparative Example 5

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:1.8:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:1.5:5:8.5.

Comparative Example 6

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:3.5:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:2.9:5:8.5.

Comparative Example 7

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:5.3:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:4.4:5:8.5.

Comparative Example 8

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:7:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:5.8:5:8.5.

Comparative Example 9

A solid electrolytic capacitor was manufactured by a similar method toExample 1 except that polymerization solutions A and B prepared in thefollowing manner were used. Specifically, polymerization solution A wasprepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:8.8:5:11.5, and polymerization solution Bwas prepared so that the ratio was3,4-ethylenedioxythiophene:γ-glycidoxypropyltrimethoxysilane:iron (III)p-toluenesulfonate:1-butanol=1:7.3:5:8.5.

Performance Evaluation

Relative Value of Capacitance

For the solid electrolytic capacitors of the Examples and ComparativeExamples each, the capacitance (μF) as an initial characteristic wasmeasured. Specifically, from solid electrolytic capacitors of theExamples and Comparative Examples, 120 solid electrolytic capacitorswere selected at random for each Example and each Comparative Example.An LCR meter of four-terminal type was used to measure the initialcapacitance (μF) at a frequency of 120 Hz of each solid electrolyticcapacitor. The average capacitance of each Example and that of eachComparative Example were calculated. The average initial capacitance ofthe solid electrolytic capacitors of Comparative Example 1 was used as areference value, and the relative value of the average initialcapacitance of the solid electrolytic capacitors, with respect to thereference value, was calculated for each Example and each ComparativeExample. The results are summarized in Table 1.

LC-Defect-Free Rate

For the solid electrolytic capacitors of the Examples and ComparativeExamples each, the LC-defect-free rate (%) was measured. LC-defect-freerate is an index representing the degree of leakage current of a solidelectrolytic capacitor. Specifically, from solid electrolytic capacitorsof the Examples and Comparative Examples, 120 solid electrolyticcapacitors were selected at random for each Example and each ComparativeExample. To each solid electrolytic capacitor, a resistor of 1 kΩ wasconnected in series and, after a rated voltage of 25V of a DC powersupply was applied for one minute, leakage current was measured. It wasdetermined that a solid electrolytic capacitor with a leakage currentamount of 37.5 μA or less was a non-defective product. TheLC-defect-free rate of the Examples and Comparative Examples each wascalculated. The results are summarized in Table 1.

ESR Value

For the solid electrolytic capacitors of the Examples and ComparativeExamples each, the ESR value (mΩ) as an initial characteristic wasmeasured. Specifically, from solid electrolytic capacitors of theExamples and Comparative Examples, 120 solid electrolytic capacitorswere selected at random for each Example. An LCR meter of four-terminaltype was used to measure the ESR value (mΩ) at a frequency of 100 Hz ofeach solid electrolytic capacitor. The average of each Example and thatof each Comparative Example were calculated. The results are summarizedin Table 1.

Rate of Occurrence of Short Circuit

For the solid electrolytic capacitors of the Examples and ComparativeExamples each, the rate of occurrence of short circuit was measured.Specifically, a hot-air reflow test, which is a reliability test, wasconducted as follows. In an environment of 121° C. or higher and 2atmospheric pressure, solid electrolytic capacitors of the Examples andComparative Examples each were left for 12 hours to cause forcedmoisture absorption, and thereafter the capacitors were kept at ahighest temperature of 260° C. for ten seconds, which was repeated fourtimes. To the solid electrolytic capacitors of the Examples andComparative Examples each having undergone the reflow test, a ratedvoltage of 25 V was applied for one minute to make a test as to whethershort circuit occurred or not. Then, the rate of occurrence of shortcircuit to solid electrolytic capacitors of the Examples and ComparativeExamples each was calculated. It was determined that short circuit hadoccurred when a leakage current of 1 mA or higher was measured. Theresults are summarized in Table 1.

TABLE 1 concentration of silane compound in relative value ofpolymerization solution (%) capacitance (with rate of polymerizationpolymerization respect to Comparative ESR occurrence of solution Asolution B Example 1) LC-defect- value short circuit (inner layer)(outer layer) (%) free rate (%) (mΩ) (%) Example 1 5 10 −2 98 75 12Example 2 10 5 −2 98 78 2.4 Example 3 15 5 −2 98 86 2.3 Example 4 20 5−2 98 98 2.3 Example 5 30 5 −3 98 110  2.3 Comparative 0.2 0.2 0  0 — —Example 1 (reference) Comparative 1 1 −1 80 62 50 Example 2 Comparative4 4 −2 95 67 25 Example 3 Comparative 5 5 −2 98 70 17 Example 4Comparative 10 10 −2 98 90 2.4 Example 5 Comparative 20 20 −2 98 120 2.3 Example 6 Comparative 30 30 −3 98 150  2.3 Example 7 Comparative 4040 −10 — — — Example 8 Comparative 50 50 −15 — — — Example 9

It has been confirmed from the results shown in Table 1 that, when theconcentration of the silane compound in the conductive polymer layer issubstantially equal to the conventional one (Comparative Examples 1 to3), the short circuit occurrence rate is higher and the LC-defect-freerate is lower, while Examples 1 to 5 and Comparative Examples 4 to 7have a lower short circuit occurrence rate and a higher LC-defect-freerate. It has also been confirmed that Comparative Examples 8 and 9 havea considerably decreased capacitance.

From a comparison of Example 1 with Comparative Examples 4 and 5 as tothe ESR value and the rate of occurrence of short circuit, it has beenfound that Example 1 is superior in terms of the characteristics of thecapacitor as a whole. It has accordingly been found that a solidelectrolytic capacitor having superior capacitor's intrinsiccharacteristics can be obtained by preparing respective polymerizationsolutions used for respective immersion processes so that theconcentration of the silane compound is different, namely theconcentration of the silane compound is varied stepwise along thethickness direction of conductive polymer layer 13.

For Example 2, conductive polymer layer 13 was formed usingpolymerization solution A having a higher concentration of the silanecompound than that of polymerization solution B, so that theconcentration of the silane compound of inner layer 13A was higher thanthat of outer layer 13B. It has been found that, in this case, thecapacitor's characteristics are superior as a whole in terms ofcapacitance, LC-defect-free rate, ESR value, and short circuitoccurrence rate. It has therefore been found that conductive polymerlayer 13 is preferably formed using a polymerization solution for afirst immersion process that is higher in concentration of the silanecompound than a polymerization solution for a second immersion process.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the scopeof the present invention being interpreted by the terms of the appendedclaims.

The invention claimed is:
 1. A solid electrolytic capacitor comprising:an anode body having a surface on which a dielectric film is formed; anda cathode layer formed on said dielectric film, said cathode layerincluding at least a first conductive polymer layer and a secondconductive polymer layer, said first conductive polymer layer and saidsecond conductive polymer layer containing a silane compound, and thesilane compound in said first conductive polymer layer and the silanecompound in said second conductive polymer layer having respectiveconcentrations different from each other.
 2. The solid electrolyticcapacitor according to claim 1, wherein said first conductive polymerlayer is located closer to said dielectric film than said secondconductive polymer layer, and the concentration of the silane compoundin said first conductive polymer layer is higher than the concentrationof the silane compound in said second conductive polymer layer.
 3. Amethod of manufacturing a solid electrolytic capacitor comprising thesteps of: forming a dielectric film on a surface of an anode body; andforming a cathode layer on a surface of the dielectric film, said stepof forming a cathode layer including at least the steps of: forming afirst conductive polymer layer by using a first treatment solutioncontaining a silane compound; and forming a second conductive polymerlayer by using a second treatment solution containing a silane compound,and the silane compound in said first treatment solution and the silanecompound in said second treatment solution having respectiveconcentrations different from each other.
 4. The method of manufacturinga solid electrolytic capacitor according to claim 3, wherein said stepof forming a first conductive polymer layer is performed prior to saidstep of forming a second conductive polymer layer, and the concentrationof the silane compound in said first treatment solution is higher thanthe concentration of the silane compound in said second treatmentsolution.
 5. The method of manufacturing a solid electrolytic capacitoraccording to claim 3, wherein the concentration of the silane compoundin said first treatment solution and the concentration of the silanecompound in said second treatment solution are not less than 5% and notmore than 30%.
 6. The method of manufacturing a solid electrolyticcapacitor according to claim 4, wherein the concentration of the silanecompound in said first treatment solution and the concentration of thesilane compound in said second treatment solution are not less than 5%and not worse than 30%.